As power supplies for driving portable electronic equipment such as cell phones, portable personal computers, and portable music players, alkaline secondary batteries represented by a nickel hydrogen battery and nonaqueous electrolyte secondary batteries represented by a lithium ion battery are widely used. Furthermore, exhaust controls of carbon dioxide gas and the like are being tightened up in view of the recent moves to protect the environment. Thus, in the car industry, not only automobiles using fossil fuels such as gasoline, diesel oil, and natural gas, but also electric vehicles (EVs) and hybrid electric vehicles (HEVs) have been developed actively. In addition, a recent sudden rise in the price of fossil fuels has accelerated the development of EVs and HEVs.
The batteries for such EVs or HEVs are required to have high output characteristics, so that each battery is upsized and a number of batteries are connected in series or in parallel to be used. In particular, the sealed batteries used for such applications need to have superior safety as compared with batteries for compact equipment because, in particular, the nonaqueous electrolyte secondary batteries use materials with very high reactivity. Thus, as shown in JP-A-2008-66254 and JP-A-2008-66255, the sealed battery used for EVs or HEVs is equipped with a gas exhaust valve for releasing internal pressure when the pressure in the battery outer can increases as well as with the current interruption mechanism for interrupting the electrical connection between an external terminal and an electrode assembly in the outer can.
That is, as shown in FIG. 6A, JP-A-2008-66254 discloses the invention of a sealed battery 50 which includes an external terminal 53 having a through-hole 52 through which a current interruption mechanism 51 communicates with the outer space of the sealed battery 50. Therefore, the current interruption mechanism 51 reliably works when the pressure in an outer can 54 increases. Furthermore, as shown in FIG. 6B, JP-A-2008-66255 discloses the invention of a sealed battery 60 which includes an external terminal 63 having a through-hole 62 through which a current interruption mechanism 61 communicates with the outer space of the sealed battery 60. Therefore, the current interruption mechanism 61 works when the pressure in an outer can 64 increases. In addition, in the sealed battery 60, the through-hole 62 is sealed with a resin film stopper 65 in order to prevent deterioration of the current interruption mechanism 61 from entering water or oxygen through the through-hole 62.
Each through-hole of the sealed batteries disclosed in JP-A-2008-66254 and JP-A-2008-66255 is installed in order that the space on the current interruption mechanism at the side corresponding to the outside of the battery communicates with the outside of the battery, so that the current interruption mechanism works easier when the pressure in the outer can increases. Thus, the resin film stopper 65 used in the sealed battery 60 disclosed in JP-A-2008-66255 needs to be thin and is not considered to be a strong stopper. Because of this, in the sealed battery 60 disclosed in JP-A-2008-66255, the film stopper 65 may be broken by any impact from the outside, and moreover, when the film stopper 65 is broken, entering water, oxygen, or the like from the outside may deteriorate the current interruption mechanism 61 as shown in JP-A-2008-66254.
However, experiments by the inventors reveal that the action of the current interruption mechanism makes substantially no difference whether the space on the current interruption mechanism at the side corresponding to the outside of the battery is closed or open. That is, even when the pressure in the outer can is increased for some reason, because the pressure of gas generated in the battery under abnormal conditions is very high, the pressure in the closed space on the current interruption mechanism at the side corresponding to the outside of the battery seldom simultaneously increases equally. Thus, there is no problem even when the space on the current interruption mechanism at the side corresponding to the outside of the battery is sealed.